二分

今日有幸見到一種二分的寫法，甚是驚奇：

LL bs(LL n, LL k) {//找到滿足結果大於等於n的最小的值
    LL l = 1, r = k + 1;
    while(l + 1 < r) {
        LL m = (l+r) >> 1;
        if( (k+m) * (k+1-m) >= 2*n ) l = m;
        else r = m;
    }
    return k+1-l;
}